Differentiation between Ca2+ transport and ATP-induced Ca2+ binding by sarcoplasmic reticulum.
The Ca2+ actively accumulated by sarcoplasmic reticulum isolated from skeletal muscle is composed of two fractions; one represented by intravesicular free Ca2+ and another represented by Ca2+ selectively bound to the membranes. Both of these Ca2+ fractions depend on ATP, although it is not clear whether ATP hydrolysis is essential for accumulation of the second Ca2+ fraction. The existence of the membrane-bound Ca2+ induced by ATP is clearly shown in experiments in which the Ca2+ retention by sarcoplasmic reticulum is measured in the presence and in the absence of X-537A, a Ca2+ ionophore, which makes the membrane permeable to Ca2+. Thus, in the presence of X-537A all Ca2+ accumulated due to ATP is bound to the membranes. This membrane-bound Ca2+ represents about 30 nmol/mg protein in the range of external pCa values of 7 to 3.5. The magnitude of this Ca2+ fraction is slightly higher whether or not the experiments are performed in the presence of oxalate, which greatly increased the intravesicular Ca2+ accumulation. Furthermore, taking advantage of the impermeability of sarcoplasmic reticulum to EGTA, it is possible to show the existence of the membrane-bound Ca2+ as a distinct fraction from that which exists intravesicularly.
D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D004533:Egtazic Acid / Q000494:pharmacology; D007700:Kinetics; D007832:Lasalocid / Q000494:pharmacology; D008565:Membrane Proteins / Q000378:metabolism; D009132:Muscles / Q000378:metabolism*; D011485:Protein Binding; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*
D008565:Membrane Proteins; D004533:Egtazic Acid; D000255:Adenosine Triphosphate; D002118:Calcium; D007832:Lasalocid